Google gave me this link while I was looking for a poster of Ralph McQuarrie's Docking Bay 94 artwork (it apparently doesn't exist outside the art for Celebration Japan) and I am flabbergasted:
http://binarybonsai.com/blog/george-lucas-stole-chewbacca-but-its-okay

It doesn't entirely surprise me as to what may have happened, after all George Harrison accidentally stole "He's So Fine" for "My Sweet Lord" without ever noticing, it happens in art from time to time, but Chewie is so iconic that it's hard to think his visual origins came from an unrelated project. Yet it does seem pretty likely.

I do wonder though, am I being generous in giving Lucas and/or McQuarrie the benefit of the doubt in this? Sure, it's all academic, but from a personal perspective it seems almost biased for my natural gut feeling to automatically assume it was a forgivable honest mistake.